This invention relates to a fuel system for an internal combustion engine and more particularly to a unit injector fuel system including a common rail capable of offering a variety of injection pressure and rate-shaping characteristics.
A fuel system is the component of an internal combustion engine which often has the greatest impact on performance and cost. Accordingly, fuel systems for internal combustion engines have received a significant portion of the total engineering effort expended to date on the development of the internal combustion engine. For this reason, today""s engine designer has an extraordinary array of choices and possible permutations of known fuel system concepts and features. Design effort typically involves extremely complex and subtle compromises among considerations such as cost, size, reliability, performance, ease of manufacture, and retrofit capability on existing engine designs.
The challenge to contemporary designers has been significantly increased by the need to respond to governmentally mandated emissions abatement standards while maintaining or improving fuel efficiency. In view of the mature nature of fuel system designs, it is extremely difficult to extract both improved engine performance and emissions abatement from further innovations in the fuel system art. Commercially competitive fuel injection systems of the future will almost certainly need to not only incorporate new design features for better achieving various objectives including improved engine performance and emissions abatement but, combine the appropriate features in the most effective manner to form a system capable of most efficiently, effectively and reliably achieving the greatest number of objectives.
Some of the most important features for achieving objectives such as improved engine performance and emissions abatement include high injection pressure capability, improved hydraulic and mechanical efficiency, quick pressure response and effective and reliable injection rate shaping capability. Other important features include drive train noise control and packaging flexibility for enabling installation on various engine configurations including retrofitting existing engines. In recent years, in an effort to provide greater operating flexibility and performance, the fuel systems industry has focused considerable attention on developing energy accumulating, nozzle controlled concepts that provide engine speed and load independent control over fuel injection timing, pressure, quantity and multiple injection rate shaping. This attention has lead to the commercialization of fuel systems packaged in the general form of a fluid pressurizing pump connected to a hydraulic energy storage device, i.e. an high pressure accumulator or common rail, connected to one or more electrically operable injector nozzles. SAE Technical Papers 960870 and 980803 both disclose common rail fuel injection systems having a high pressure common rail containing high pressure fuel for delivery to multiple injectors. Each injector includes a servo-controlled needle valve element assembly. The assembly includes a control volume in communication with an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve. U.S. Pat. No. 5,133,645 to Crowley et al. discloses a similar system. These systems all permit speed and load independent pressure control, a broad fuel injection timing range, and fast injection timing, quantity, and multiple pulse rate shaping transient response. However, in these systems, fuel injection pressure is varied by changing the pressure in the entire common rail making fuel pressure responsiveness difficult to achieve. Moreover, the large volume of fuel at high pressure requires more expensive seals along with greater risk of leakage and repair costs.
Another conventional approach is disclosed in U.S. Pat. Nos. 5,535,723 and 5,551,398 and SAE Technical Paper 961285 which disclose a unit fuel injector system including a mechanically actuated plunger in each injector and a servo-controlled needle valve element assembly. Fuel is supplied to each injector at a low supply pressure and pressurized by the plunger to a very high injection pressure. Each of these systems also includes a pump control valve for controlling the flow out of a pumping chamber to control the pressurization of the fuel to a high pressure level. These systems advantageously permit very high injection pressures. U.S. Pat. No. 5,463,996 issued to Maley et al. discloses a similar system wherein the high pressure fuel for injection is generated by a hydraulically driven intensifier plunger.
Consequently, there is a need for a high pressure fuel system for an internal combustion engine which is capable of providing enhanced operating flexibility and performance with respect to at least injection pressure and rate shaping while protecting investments in existing engine architectures.
It is an object of the present invention, therefore, to overcome the disadvantages of the prior art and to provide a fuel injection system capable of effectively and predictably controlling fuel injection timing and metering.
It is another object of the present invention to provide a fuel injection system capable of controlling fuel injection pressure independent from engine speed.
It is yet another object of the present invention to provide a fuel injection system capable of providing greater operating flexibility and performance.
It is a further object of the present invention to provide a fuel injection system capable of providing a wide range of selectable injection pressure levels for creating multiple injections including pilot and/or post injections in combination with a main injection event.
It is a still further object of the present invention to provide a highly efficient high pressure fuel injection system capable of recuperating the pressure energy stored in the pressurized fuel in the common rail following an injection event.
Yet another object of the present invention is to provide a fuel injection system which protects investments in existing engine architectures by being easy to retrofit on existing mechanically operated unit injector-equipped engines.
Still another object of the present invention is to provide a fuel injection system capable of providing extremely high pressures main injections while permitting lower pressure pilot injections.
A still further object of the present invention is to provide a fuel injection system capable of combining the desirable features of mechanically operated unit injectors and contemporary common rail concepts in packaging that is cost effective.
Yet another object of the present invention is to provide a fuel injection system which provides rapid pressure response, improved small injection metering repeatability and accuracy, improved limp home and fail safe functionality and energy recovery for improved operating efficiency.
Another object of the present invention is to provide a fuel system capable of high pressure injections, triangular rising rate shape and high modularity and redundancy.
It is yet another object of the present invention to provide a high pressure fuel injection system capable of a broad fuel injection timing range supporting pilot, main, post and after treatment needs; speed and load independent pressure control; low noise, vibration and harshness (NVH); and high operating efficiency.
It is still another object of the present invention to provide a high pressure fuel injection system capable of fast injection timing, quantity, pressure and multiple pulse rate shaping transient response.
A still further object of the present invention is to provide a fuel system capable of producing a variety of different combinations of discrete and blended pilot, main, post and after treatment fuel injection events during a single combustion cycle.
These and other objects are achieved by providing a fuel injection system for controlling fuel injection into combustion chambers of a multi-cylinder internal combustion engine, comprising a low pressure fuel supply for supplying fuel at a low supply pressure level, a common rail fluidically connected to the low pressure fuel supply and containing injection fuel at a common rail pressure level greater than the low supply pressure level, and a plurality of fuel injectors connected to the common rail to receive fuel from the common rail and inject fuel at a high pressure level greater than the common rail pressure level. Each of the plurality of injectors includes an injector body containing an injector cavity, a fuel transfer circuit and an injection orifice formed at one end of the injector body, a plunger reciprocally mounted in the injector cavity, and a high pressure chamber formed between the plunger and the injection orifice. The plunger is movable into the high pressure chamber to increase the pressure of fuel in the high pressure chamber to a high pressure level greater than the common rail pressure level. Each injector also includes a nozzle assembly including a needle valve element reciprocally mounted for movement between a closed position blocking fuel flow through the injection orifice and an open position permitting fuel flow through the injection orifice. Each fuel injector further includes a needle valve control device adapted to move the needle valve element between the closed and open positions to initiate an injection event to inject the fuel at the common rail pressure when the injection system is operating in a first mode and at the high pressure level when the injection system is operating in a third mode. The needle valve control device includes a control volume, a drain circuit for draining fuel from the control volume to a low pressure drain and an injection control valve positioned along the drain circuit for controlling the flow fuel through the drain circuit so as to cause the movement of the needle valve element between the open and the closed positions. The fuel injection system further includes a pump control valve for controlling the flow of fuel into the high pressure chamber.
Preferably, the common rail pressure is greater than approximately 15 MPa and, more specifically, in the range of approximately 20-50 MPa. The system may further include a common rail pump for receiving low pressure fuel from the low pressure fuel supply and delivering the fuel to the common rail at at least the common rail pressure level. The system may further include an accumulator connected to the common rail and a pressure limiter for directing high pressure fuel to the common rail during the third mode to prevent overpressurization. The fuel injection system may further include a variable inlet metering orifice positioned upstream of the common rail pump and a low pressure regulator positioned between the low pressure fuel supply and the variable inlet metering orifice.
In one embodiment, the fuel injection system may include a first branch circuit extending from the low pressure fuel supply and a second branch circuit extending from the low pressure fuel supply in parallel to the first branch circuit wherein the common rail is positioned along the first branch circuit and the pump control valve is positioned to receive fuel from the second branch circuit. In this embodiment, a check valve may be positioned along the fuel transfer circuit to permit fuel flow from the common rail to the closed nozzle assembly while preventing flow from the common rail to the high pressure chamber when the injection system is operating in the first mode and permitting high pressure fuel flow from the high pressure chamber to the nozzle valve assembly while preventing fuel flow from the common rail to the nozzle valve assembly when the injection system is operating in the third mode.
In multiple embodiments, the plunger is movable to spill fuel from the high pressure chamber through the pump control valve into one of a supply rail and a common rail. In these embodiments, the injection system is operable in a second mode to move the needle valve element between the closed and the open positions to initiate an injection event to inject fuel during spilling of fuel from the high pressure chamber. The plunger may be movable through a retraction stroke to refill the high pressure chamber with fuel. In this case, the injection system is operable in a fourth mode to move the needle valve element between the closed and the open positions to initiate an injection event to inject fuel during refill of the high pressure chamber. The low pressure fuel supply may include a low pressure supply pump and a supply rail positioned in parallel with the common rail for receiving fuel from the low pressure supply pump. In one embodiment, the high pressure chamber is fluidically connected to the supply rail so as to receive low pressure fuel directly from the supply rail and fluidically connect the common rail to receive fuel at common rail pressure directly from the common rail via the pump control valve. A check valve may be positioned between the supply rail and the high pressure chamber to permit fuel flow from the supply rail to the high pressure chamber while preventing fuel flow from the high pressure chamber to the supply rail. The injection system may be operable in a common rail recharge mode during which the plunger moves at least partially through the retraction stroke while the pump control valve is maintained in a closed position to cause fuel flow from the supply rail through the check valve into the high pressure chamber.